Mode suppression in fluid meter conduits

ABSTRACT

The present invention relates to a fluid flow arrangement where the effects of high order acoustic modes are suppressed in ultrasonic flow measurement. In one embodiment, ultrasonic transducers are arranged in a duct configured to carry a fluid. Ultrasonic signals between the transducers travel in a plurality of high order modes having velocities slower than a fundamental mode whose time of flight is desired to determine with accuracy. A measurement portion between the transducers is adapted to suppress the propagation of the non-fundamental modes.

FIELD OF THE INVENTION

The present invention relates generally to ultrasonic flow measurementsof fluids and, in particular, to the controlling and/or suppression ofthe effects, on the measurements, of non-fundamental acoustic modes.

BACKGROUND ART

It is known that the velocity of the flow of a gas as well as thevelocity of sound in the gas can be determined by the taking of twomeasurements of the transit time of a pulse of ultrasound, one upstreamand one downstream in the flowing stream. This is the principle ofoperation of a transit time ultrasonic gas meter. In order to performthis timing with small uncertainty, it is necessary to select someprecise feature of the pulse which can serve as a timing marker. Thecrossing of the signal through zero is capable of very accurate locationin time and makes a good timing marker. There are, however, a number ofzero-crossings from which one such crossing must be reliably selected asthe timing marker.

FIG. 1E shows a typical ultrasonic measurement arrangement 1 in whichtwo transducers 2,3 face each other through a duct 4 having acylindrical shape and circular cross-section with gas flowing in thedirection indicated by the arrow 6.

FIGS. 1A-1D show a signal typically received from a pulse of ultrasoundlaunched into the circular duct 4. The received signal is initiallyshown in FIG. 1A and extends for some time shown in FIGS. 1B, 1C and 1Dwhere markers indicate times, 2 times, 3 times and 4 times respectivelypast the initial arrival of the start of the signal.

A particular negative going zero-crossing has been chosen as the eventon which to do the timing. International Patent Application No.PCT/AU92/00314 (WO 93/00569) discloses an electronic fluid flow meterwhich incorporates circuitry developed which select this particularcrossing in a two-stage process. Using the signal amplitude, a time ischosen before the arrival of the required crossing at which anegative-going zero-crossing detector is enabled. There is a reasonablelatitude allowable in the timing for this preliminary event but it isapparent that it must occur before the chosen zero-crossing but afterthe preceding one. Otherwise, it would be used instead for the precisetiming marker.

Such an arrangement has been found to operate satisfactorily. As it isbased on the amplitude of the signal and how it varies with time,anything which changes the amplitude of the signal has the potential tointerfere with the choice of the initial time and thus in the selectionof the correct zero-crossing. One cause of amplitude changes inelectronic systems are gain changes. These can be due to aging,temperature, or other environmental effects. It is normal to compensatefor such changes by some form of automatic gain control (AGC) whichalmost eliminates these amplitude changes. There are, however, othercauses of amplitude change that affect the individual peak heights inthe signal even when, as a result of the AGC, the maximum peak height isconstant. Furthermore, when amplitude changes cause an incorrect choiceof zero-crossing the timing error which results will be at least onewhole period of the signal. This represents a serious error because itis systematic and averaging will not produce an unbiased mean of smalleruncertainty. A significant contributor to these amplitude changes is theexistence and propagation of, non-fundamental acoustic modes.

SUMMARY OF THE INVENTION

It is an object of the present invention to substantially overcome, orameliorate, the abovementioned problems through provision of a means bywhich the effects of non-fundamental acoustic modes on the receivedsignal, typically the amplitude thereof, can be further reduced.

Throughout the specification reference to "non-fundamental acousticmodes" is to be taken as including a reference to higher order acousticmodes, and vice versa.

In accordance with one aspect of the present invention there isdisclosed a fluid flow measurement system comprising a duct for carryinga fluid, the duct having two transducers spaced apart therein to definea fluid flow measurement portion therebetween, wherein the measurementportion is adapted to control effects of non-fundamental acoustic modeson a signal received by at least one of the transducers.

Generally, the measurement portion is configured to control thepropagation between the transducers of the non-fundamental acousticmodes, in order to reduce their effect as received by the transducer(s),in which the propagation is suppressed by decreasing the speed withwhich the non-fundamental acoustic modes travel or by modifying thephase relationships between them.

Advantageously, the speed of the fundamental acoustic mode is notdecreased, and the effects of the non-fundamental acoustic modes isreduced by decreasing their net contribution upon a signal amplitudereceived by the one transducer.

In one form, the portion has at least one obstruction located in theduct whereby fluid in the duct can flow thereabout, the obstructionhaving a shape and being so located within the duct to suppress effectsof the non-fundamental acoustic modes. Generally, the obstruction ispositioned centrally within the duct. Alternatively, the obstruction(s)can be disposed non-centrally within the duct. Advantageously, theobstruction(s) is fluid dynamically shaped to reduce and ideallyminimise a decrease in fluid pressure thereabout. Generally, this isachieved by locating an appropriate fluid dynamically shapedobstruction(s) having a substantially smooth surface within themeasurement portion of the duct so as to minimise or substantiallyreduce fluid pressure drop within the duct.

In a particular form, at least a portion of the measurement portionitself is shaped to have an augmented, non-circular cross-section.Typically this is achieved by the duct having at least two wall portionsinterconnecting wherein one of the wall portions is curved and the otheris substantially non-curved. The curved wall portion may be a partialellipse, circle, parabola, hyperbola, cycloid, hypocycloid, or anepicycloid. The interconnection between the portions may be integrallyformed, or formed from separate substrates. The obstruction, asindicated above, is located within the augmented measurement portion.

In one typical fluid flow measurement system there is a duct forcarrying a fluid and having therein two transducers spaced apart todefine a measurement portion therebetween, the measurement portioncomprising a cylindrical duct and at least one fluid dynamically shapedobstruction located centrally therein.

Alternatively, the measurement portion can include at least a portion ofthe measurement portion having a shape which is non-circular incross-section in combination with at least one fluid dynamically shapedobstruction disposed in the measurement portion, the combination beingadapted to suppress effects of non-fundamental acoustic modes receivedby at least one of the transducers.

Generally, the obstruction is an ellipsoid and disposed between thetransducers centrally in the duct along its principal axis, therebyresulting in the cross-section being annular about the obstruction andwherein the width of the flow passage between the duct walls and theobstruction varies along the obstruction.

There may be one, two, three up to ten or more obstructions disposed inthe duct. The inner surface of the duct forming the measurement portioncan also be roughened to suppress effects of non-fundamental acousticmodes received by at least one of the transducers.

A system in accordance wth the above can be used to take fluid flowmeasurements for either liquids or gases. Generally, the gas can bedomestic gas, methane, propane, oxygen, hydrogen or industrially usefulgases. Advantageously, the system can be configured as part of adomestic or industrial gas meter specifically adapted for measuring theflow of so-called "natural gas". Applications for liquid flowmeasurement include liquid hydrocarbon and water metering, as well as aships' log.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of embodiments of the present invention will now be describedwith reference to the drawings in which:

FIGS. 1A to 1D depict the reception of an acoustic pulse in thearrangement of FIG. 1E;

FIG. 2 depicts the fundamental (0,1) and various high order acousticmodes present in a circular duct of FIG. 1E;

FIG. 3 illustrates a converging flow and the (0,2) modes;

FIG. 4 shows the received waveform of FIG. 3;

FIG. 5 is a view similar to FIG. 4 but after application of oneembodiment of the invention;

FIG. 6 depicts a single obstruction embodiment used in FIG. 5;

FIG. 7 shows an alternative embodiment which includes two obstructions;

FIG. 8 shows waveforms resulting from the embodiment of FIG. 7;

FIGS. 9A-9D are views similar to FIGS. 1A to 1D but of a tubeincorporating an obstruction such as that of FIG. 6 or FIG. 7;

FIG. 10 shows the effects of temperature variations of the gas tube;

FIGS. 11A and 11B show signals transmitted in the tube of FIG. 10;

FIG. 12 illustrates the improvement over FIG. 11 using anotherembodiment of the invention;

FIGS. 13A and 13B compare waveforms for temperature variation with andwithout an obstruction.

FIG. 14 depicts the transmission of ultrasound using the ring-aroundtechnique;

FIGS. 15A and 15B illustrate two tube shapes representing furtherembodiments which reduce the effects of high order modes resulting fromthe use of the ring-around technique;

FIGS. 16A-16D are views similar to FIGS. 1A to 1D but resulting from thecombination of FIGS. 6 and 15A; and

BEST AND OTHER MODES FOR CARRYING OUT THE INVENTION

When a pulse of sound is launched down a tube or duct, it excites anumber of acoustic modes that carry the signal. The modes may simply bethought of as reflections from the wall of the tube or may be consideredas the vibrations of the stretched skin of a drum but propagated throughspace. Both of these analogies have their uses but each is limited.

The reflection analogy enables the determination that the velocity ofpropagation along the tube of the modes which are reflected more times,will be slower. The fastest mode is the plane wave or fundamental mode,which is not reflected. The plane wave mode propagates along the tubewith the speed of sound in free space, denoted by c. The other modes,because of their reflection(s) from the tube wall, propagate along thetube at a wide range of velocities from almost c down to zero, thoughstrictly speaking, a mode with zero velocity does not propagate. Ingeneral the slower modes are less excited, and therefore have smalleramplitudes, than the faster modes. For modes having propagationvelocities less than c/5, the amplitude generally becomes only one ortwo percent of the main amplitude. Each mode except for the plane wavehas a cut-off frequency below which it will not propagate in thatparticular duct. For the frequency and duct size involved, there arevery many modes above cut-off.

Whilst the modes are separate entities, they superimpose to give analmost continuous signal. Some modes are more strongly received thanothers and so the total signal has a lumpy appearance, as shown in FIGS.1A-1D, from which it will be seen that there are modes that areconsiderably larger than the modes of similar velocity on either side.

FIG. 2 shows a number of representations, using the drum head analogy,of some of the faster modes present in the circular duct 4. The (0,2)mode is a vibration of the central portion out of phase with theperimeter. This is expected because the transducers 2,3 act in thecentral part of the duct 4 where they cause strong excitation. Thevelocity of the (0,2) mode is only slightly slower than that of theplane wave (0,1) mode so that the signal received is an addition ofthose two modes, unless a very long tube is used which allows the modesto separate. For a cylindrical duct the exact phase relationship of thetwo signals determines the magnitude of the received composite signal.The phase relationship depends on the diameter of the duct 4, the lengthof the duct (between the transducers 2,3), and on the plane wavevelocity.

The velocity of sound, the plane wave velocity, is given by ##EQU1##where γ is the ratio of specific heats, P is the pressure and ρ_(O) isthe density of the gas. Thus the velocity will depend on the nature ofthe gas and, for a particular gas at a fixed pressure and an absolutetemperature T, the velocity will be given in terms of the velocity atsome standard temperature, in this case 273K, by ##EQU2##

A change in the value of C alters the phase relationship and can alterthe magnitude of the composite signal considerably. This amplitude canalso be influenced by a converging flow.

Similarly, transmission into a converging flow is shown in FIG. 3. Thewaveform received in such a situation is shown in FIG. 4 where it willbe seen that the second maximum in the envelope is larger than thefirst. One possible explanation for this is that the converging velocityfield bends the sound from the transducer 3 which would otherwise belost into a region on the perimeter of the smaller diameter section. Asimilar effect can be seen with hot gas in a cold tube (to be discussedlater). This makes the detection of a particular zero-crossingsubstantially more difficult.

Turning now to FIG. 6, if a central obstruction 10 is placed in the duct4, then the central part of the (0,2) mode, represented by referencenumeral 5, and other more complicated modes which have a centralcomponent, are prevented from propagating. In such a case, the duct 4about the obstruction 10 has an annular cross-section, which in view ofthe aerodynamic shape of the obstructions 10, varies along its length.The effect of this can be seen in FIGS. 4 and 5 that show the signalreceived through a duct 4 before and after the addition of a centralobstruction 10 respectively. FIG. 4 shows two waveforms, one at zeroflow in which the (0,2) mode is about two thirds the amplitude of theplane wave mode, and one for downstream flow in which the ratio of theamplitudes is reversed. In FIG. 5, which has the central obstruction 10,the size of the (0,2) mode is much reduced and the waveforms for zeroflow and downstream flow are very similar.

The placement of the central obstruction 10 is not critical, but it hasbeen found that a position about four or five diameters of the duct 4and in from the entrance of the duct 4, as indicated in FIG. 6, givesthe best results. The shape of the obstruction is governed byaerodynamic reasons and a sphere or tear drop is suitable. The optimaldiameter of the obstruction 10 is about one half the diameter of theduct 4, but again this is not a critical dimension.

More than one obstruction 10 can be used to some additional benefit.FIG. 7 shows a duct 13 with two tear drop shaped obstructions 11 and 12.The duct 13 is insensitive to the direction of the flow and to thenature of the gas as regards the initial waveform. An example of thereceived signal in the cases of a downstream flow of air (as for FIG. 4)and a zero flow of natural gas is shown in FIG. 8. The wave shape forother combinations of gas type and flow also produce little change inthe received waveform.

FIGS. 9A-9D show the higher modes for a cylindrical tube fitted with acentral obstruction 10 as in FIG. 6. The amplitude of the higher modeshas been diminished and of course the behaviour of the (0,2) mode hasbeen controlled.

When the temperature of the meter body is very much less than that ofthe gas either because the meter body is very cold, or the meter body isat room temperature and the gas is very hot, then the (0,2) mode can beexcited more than normal. A reason for this is illustrated in FIG. 10.Here, a hot gas flow 6 contacts the cold wall of the duct 4 to produce acold layer of gas 7. Energy which would normally be reflected out of theduct 4 and hence, in a sense wasted, as is depicted by the arrow 9, isable to enter the duct 4 because it is refracted by the layer of coldgas 7 as depicted by the arrow 8. From equation 2 above, the velocity ofsound in a gas is less if the gas is cold than if the gas is hot. A beamof sound entering a region of cold gas is bent as shown in the FIG. 10,in much the same way as a beam of light is refracted in an opticallydense medium. The effect on the received signal can be seen in FIGS. 11Aand 11B for a hot gas flowing in a cold duct.

FIG. 11A represents an upstream transmission and FIG. 11B a downstreamtransmission. Compared with waveforms for zero flow at a uniformtemperature, the (0,2) mode has been decreased in FIG. 11A and increasedin FIG. 11B.

Given that the above explanation is generally correct, the effect can bereduced by removing the layer of cold gas 7 at the meter wall. This canbe performed using a gas stirring device which is fitted to the inlet ofthe inlet of the duct 4 to impart into the stream of gas, a swirlingmotion to induce mixing at the wall. The effect of this device can beseen by comparing FIGS. 11B and 12. In FIG. 12 the stirring device wasused but otherwise the experimental arrangement was identical with thatof FIG. 11B. The reduction of the (0,2) mode is clear.

A central obstruction is very effective in supression of the (0,2) modefrom this source. This is illustrated in FIGS. 13A and 13B where FIG.13A is for a tube without a central obstruction. The duct has roomtemperature gas flowing through it and the walls have been cooled with arefrigerant. The accentuation of the (0,2) mode is again apparent. FIG.13B is for the same conditions except that a central obstruction hasbeen fitted. The waveform is now normal in the sense that it veryclosely resembles the waveform for the same duct at room temperature(FIG. 5).

Though the discussion has so far been concerned with the (0,2) modethere are very many other modes. These are generally of smalleramplitude than the (0,2) mode but in certain circumstances they cancause difficulties by corrupting the propagation of the main acousticwave packet. This is particularly so in the use of the ring-aroundtechnique, described in detail in the aforementioned InternationalPatent Application No. PCT/AU92/00314.

In this technique, when a signal is detected, a new pulse is immediatelytransmitted. Because of the very long tail of higher order modes on thesignal, this retransmission arrives at the receiver while the previoussignal still has a significant amplitude. In effect, there is a layeringof signals with contributions from the higher modes of a number ofpreviously transmitted pulses. These higher modes will add to the planewave mode and produce a resultant which is the signal upon which thetiming of the next transmission is based.

The first part of the first received signal will be free of all othermodes; the second received signal will contain the plane wave mode fromthe second transmission and those modes which travel at one half thespeed of the plane wave, i.e. c/2, from the first transmission. Thethird received signal will be the sum of the plane wave from the thirdtransmission, modes with velocity c/2 from the second transmission andthose with velocity c/3 from the first transmission. The extension tothe fourth, fifth and so on received pulses will be apparent to theskilled addressee. This addition process is illustrated in FIG. 12.Furthermore, with reference to FIG. 1, the amplitudes become small asthe mode order becomes high so that there is little effect on thereceived signal from modes of velocity less than c/4.

It can be shown theoretically or seen experimentally that the effect offlow on the received signal is to move the received signal as a wholealong the time axis so that its shape is, in the main, preserved. Thearrival time of the particular zero-crossing that has been chosen as thetiming marker will vary with the flow. The arrival time will be longeror shorter depending on whether the transmission of the signal isupstream or downstream. The particular parts of the long tail of thereceived signal that add in the layering process described above will bethose parts of the tail that are integral multiples of this time afterthe selected zero-crossing. The exact combination will thus vary withthe flow.

This addition of other (high order) modes, whose phases vary with flow,to the plane wave, in the ring-around technique alters the time of thezero-crossing from that for the plane wave alone, that is, from thatthat would apply for a single transmission. When flows are calculatedfrom transit times derived from the ring-around process the high ordermodes cause a periodic deviation from a straight line response as theflow varies over the range from no flow to maximum. This problem hasbeen addressed by the transmission of an inverted pulse once in fourtransmissions as described in International Patent Application No.PCT/AU92/00315 (WO 93/00570) entitled "Mode Suppression in Fluid FlowMeasurements" by the same Applicants.

However, this does not prevent the amplitude of the received signal fromchanging. When an inverted pulse is transmitted, the signal from highermodes which previously had been adding to the amplitude of the planewave signal, will be subtracted from the plane wave. Thus the detectionsystem must be able to detect reliably the correct zero-crossing whenthe amplitudes of the early peaks in the received signal are altered byamounts corresponding to those higher modes. It is therefore generallyeasier for the detection circuit to operate effectively if the magnitudeof the higher modes, with velocities approximately c/2, c/3, and so on,are as small as possible.

Through examination of the nature of the modes as shown in FIG. 2, theyare all seen to possess a high degree of symmetry based on the circularcross-section of the metering tube. Thus, in order to minimise the highorder modes, it was observed necessary to break or change the symmetryof the geometry of the tube as much as possible.

FIGS. 15A and 15B show two cross-sections of metering ducts thatrepresent a compromise between the above criteria. FIG. 15A shows across-section of a duct 20 which include a substantially semi-circularcurved portion 21 and two flat faces 22 and 23. FIG. 15B shows a duct 25having a part elliptical curved portion 26 and a single flat face 27.The ducts 20 and 25 operate on the principle that high order modes tendto be reflected from the wall of the duct and in each case, modesreflected off the flat faces 22 and 23, or 27 are reflected onto, anddissipated on the curved faces 21 and 26 respectively which, in view ofthe greater circumferential length (and their surface area), reduce thesound pressure level and hence amplitude of the high order modes.

FIGS. 16A-16D show the deformed cross-section duct 20 mentioned above,which has been fitted with a single central obstruction in the form of atear drop. The downstream behaviour is now greatly improved. The use ofcentral obstructions reduces the amplitudes of some of the higher modesas well as that of the (0,2) mode. The use of the two techniquestogether, non-cylindrical forms and central obstructions, gives animproved performance as regards the higher modes.

In a further configuration, the walls of the measuring tube 4 areroughened using grooves and/or bumps which are comparable in size tohalf the wavelength of the acoustic signal, generally between 0.01 and 8millimeters for the broader ultrasonic frequencies and preferablybetween about 0.25 and 2 millimeters for the frequencies able to beemployed using the apparatus disclosed in the aforementionedInternational Patent Applications. Such a roughening was foundsufficient to reduce the contribution of higher order acoustic modeswhilst permitting the plane wave mode to dominate at all flows andtemperatures, without appreciably increasing the frictional resistanceto flow (the pressure drop) within the tube 4.

The appropriate roughened surface finish to achieve high order modedamping can be provided by casting from an appropriate mould or byconfiguring the tube wall with a helical groove of pitch approximatingthe acoustic half wave length.

The effect of a particular higher mode is reduced because the energy itcontains is spread over a finite time. That is, if the mode is reflectedoff a perfectly cylindrical surface, all of the mode front arrives atthe transducer simultaneously and its full effect is felt. Roughening ordistorting the surface causes a proportion of the mode front to travelover a slightly longer distance thereby reducing the total instantaneouscontribution. Excessive roughening of the tube surface although reducingthe influence of high order modes, can significantly increase thepressure drop.

It will be apparent from the foregoing that the addition of theobstruction, changes of the duct cross-section from a regular shape,and/or roughening/grooving of the duct between the transducers, acts todecrease the speed of propagation of the high-order acoustic modeswithout decreasing the speed of propagation path of the fundamentalmode.

One significant advantage of the mode suppression techniques describedherein is that they allow an electronic gas meter incorporating at leastone of the described arrangements, to be calibrated using air prior toinsertion into a gas pipeline. With traditional, prior art arrangemets,this technique has lead to mis-calibration due to the modal response ofultrasound in a combustible gas, such as "natural gas", being differentto that of air. In such situations, the different modal response resultsin a different time of detection of the received burst of ultrasound.With the modal reponse substantially reduced in accordance with theprinciples described herein, the detection times are consistent forgases.

The foregoing describes only a number of embodiments of the presentinvention and modifications, obvious to those skilled in the art can bemade thereto without departing from the scope of the present invention.

I claim:
 1. A fluid flow measurement system comprising a duct forcarrying a fluid, the duct having two transducers spaced apart thereinto define a fluid flow measurement portion therebetween, wherein themeasurement portion is adapted to control effects of non-fundamentalacoustic modes on a signal received by at least one of the transducerswithout substantially altering an effect of a fundamental acoustic modeon said signal; andwherein said measurement portion has at least oneobstruction located in the duct whereby fluid in the duct can flowthereabout, the obstruction having a shape and being so located withinthe duct to control effects of the non-fundamental acoustic modes.
 2. Asystem as claimed in claim 1 wherein the measurement portion is adaptedto control effects of non-fundamental acoustic modes originating fromone of said transducers on a signal received by the other of saidtransducers.
 3. A system as claimed in claim 2 wherein said measurementportion is configured to control the propagation between saidtransducers of said non-fundamental acoustic modes, to thereby reducetheir effect as received by said one transducer, said propagation beingsuppressed by decreasing the speed of said non-fundamental acousticmodes or by modifying their phase relationships.
 4. A system as claimedin claim 3 wherein the speed of the fundamental acoustic mode is notdecreased, and the effects of the non-fundamental acoustic modes isreduced by decreasing their net contribution upon a signal amplitudereceived by said one transducer.
 5. A system as claimed in claim 1,wherein each said obstruction is positioned centrally within the duct.6. A system as claimed in claim 1, wherein each said obstruction isfluid dynamically shaped to reduce a change in fluid pressurethereabout.
 7. A fluid flow measurement system comprising a duct forcarrying a fluid, the duct having two transducers spaced apart thereinto define a fluid flow measurement portion therebetween, wherein themeasurement portion is adapted to control effects of non-fundamentalacoustic modes on a signal received by at least one of thetransducers;wherein said measurement portion has at least oneobstruction located in the duct whereby fluid in the duct can flowthereabout, the obstruction having a shape and being so located withinthe duct to control effects of the non-fundamental acoustic modes;wherein each said obstruction is fluid dynamically shaped to reduce achange in fluid pressure thereabout; and wherein each said obstructionhas a substantially smooth surface within the measurement portion and isconfigured to substantially reduce fluid pressure drop within themeasurement portion.
 8. A system as claimed In claim 7, wherein saidobstruction is selected from the group consisting of an ellipsoid, andan object formed from a cone and a hemisphere having its circular facejoined to the base of said cone.
 9. A fluid flow measurement systemcomprising a duct for carrying a fluid, the duct having two transducersspaced apart therein to define a fluid flow measurement portiontherebetween, wherein the measurement portion is adapted to controleffects of non-fundamental acoustic modes on a signal received by atleast one of the transducers; andwherein at least a section of themeasurement portion is shaped to have an augmented, non-circularcross-section configured to control effects of the non-fundamentalacoustic modes.
 10. A system as claimed in claim 9, wherein said duct atsaid section of said measurement portion has at least twointerconnecting wall portions, at least one of the wall portions beingcurved, and at least one other wall portion being substantiallynon-curved.
 11. A system as claimed in claim 10, wherein the curved wallportion(s) are selected from the group consisting of a partial ellipse,a partial circle, a parabola, a hyperbola, a cycloid, a hypocycloid, andan epicycloid, and the substantially non-curved wall portion(s) arestraight.
 12. A system as claimed in claim 11, wherein interconnectionsbetween the wall portions are integrally formed.
 13. A system as claimedin claim 9, further comprising at least one obstruction located in saidmeasurement portion whereby fluid in the duct can flow thereabout, theobstruction having a shape and being so located within the duct tocontrol effects of the non-fundamental acoustic modes.
 14. A system asclaimed in claim 13, wherein at least one of said obstructions islocated within said section of said measurement portion.
 15. A system asclaimed in claim 9 wherein said duct at said section has an interiorsurface that includes irregularities configured to suppress effects ofsaid non-fundamental acoustic modes.
 16. A system as claimed in claim 15wherein said irregularities comprise grooves formed in, or a rougheningof, said interior surface.
 17. A fluid flow measurement systemcomprising a duct for carrying a fluid, the duct having two transducersspaced apart therein to define a fluid flow measurement portiontherebetween, wherein the measurement portion is adapted to controleffects of non-fundamental acoustic modes on a signal received by atleast one of the transducers; andwherein the measurement portioncomprises a cylindrical duct and at least one fluid dynamically shapedobstruction located centrally therein.
 18. A system as claimed in claim17, wherein said obstruction(s) is an ellipsoid and disposed between thetransducers centrally in the duct along its principal axis, therebyresulting in the cross-section being annular about the obstruction andwherein the width of the flow passage between the duct walls and theobstruction varies along the obstruction.
 19. A system as claimed inclaim 17, wherein an inner surface of the duct forming the measurementportion is roughened, said roughened inner surface acting to controleffects of non-fundamental acoustic modes received by at least one ofthe transducers.
 20. A fluid flow measurement system comprising a ductfor carrying a fluid, the duct having two transducers spaced aparttherein to define a fluid flow measurement portion therebetween whereinthe measurement portion is adapted to control effects of non-fundamentalacoustic modes on a signal received by at least one of the transducers;andwherein the measurement portion has a shape which is non-circular incross-section in combination with at least one fluid dynamically shapedobstruction disposed in the measurement portion, the combination beingadapted to control effects of non-fundamental acoustic modes received byat least one of the transducers.